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1. List the two broad classifications of
oscillators (wave generators).

2. Identify the three frequency-determining devices for sine-wave
oscillators.

3. Describe the differences between series-fed and shunt-fed oscillators.

4.
Explain how the crystal is equivalent to the series and parallel LC circuit.

5. Identify the Armstrong oscillator.

6. Identify the Hartley oscillator.

7. Identify the Colpitts oscillator.

8. Identify the resistive-capacitive oscillator.

9. Determine the frequency of a resistive-capacitive oscillator.

10. Explain the operation of a pulsed
oscillator.

11. Determine how many cycles are present in the output of a pulsed oscillator.

12. Explain how frequency multiplication takes place.

INTRODUCTION

WAVE GENERATORS play a prominent role in the field of electronics. They generate signals from a few
hertz to several gigahertz (109 hertz). Modern wave generators use many different circuits and
generate such outputs as SINUSOIDAL, SQUARE, RECTANGULAR, SAWTOOTH, and TRAPEZOIDAL waveshapes. These waveshapes
serve many useful purposes in the electronic circuits you will be studying. For example, they are used extensively
throughout the television receiver to reproduce both picture and sound.

One type of wave generator is
known as an OSCILLATOR. An oscillator can be regarded as an amplifier which provides its own input signal.
Oscillators are classified according to the waveshapes they produce and the requirements needed for them to
produce oscillations.

CLASSIFICATION OF OSCILLATORS (GENERATORS)

Wave generators can be classified into two
broad categories according to their output waveshapes, SINUSOIDAL and NONSINUSOIDAL.

2-1

Sinusoidal Oscillators

A sinusoidal oscillator produces a sine-wave output
signal. Ideally, the output signal is of constant amplitude with no variation in frequency. Actually, something
less than this is usually obtained. The degree to which the ideal is approached depends upon such factors as class
of amplifier operation, amplifier characteristics, frequency stability, and amplitude stability.

Sine-wave
generators produce signals ranging from low audio frequencies to ultrahigh radio and microwave frequencies. Many
low-frequency generators use resistors and capacitors to form their frequency-determining networks and are
referred to as RC OSCILLATORS. They are widely used in the audio-frequency range.

Another type of
sine-wave generator uses inductors and capacitors for its frequency-determining network. This type is known as the
LC OSCILLATOR. LC oscillators, which use tank circuits, are commonly used for the higher radio frequencies. They
are not suitable for use as extremely low-frequency oscillators because the inductors and capacitors would be
large in size, heavy, and costly to manufacture.

A third type of sine-wave generator is the CRYSTAL-CONTROLLED OSCILLATOR. The crystal- controlled oscillator
provides excellent frequency stability and is used from the middle of the audio range through the radio frequency
range.

Nonsinusoidal Oscillators

Nonsinusoidal oscillators generate complex
waveforms, such as square, rectangular, trigger, sawtooth, or trapezoidal. Because their outputs are generally
characterized by a sudden change, or relaxation, they are often referred to as RELAXATION OSCILLATORS. The signal
frequency of these oscillators is usually governed by the charge or discharge time of a capacitor in series with a
resistor. Some types, however, contain inductors that affect the output frequency. Thus, like sinusoidal
oscillators, both RC and LC networks are used for determining the frequency of oscillation. Within this category
of nonsinusoidal oscillators are MULTIVIBRATORS, BLOCKING OSCILLATORS, SAWTOOTH GENERATORS, and TRAPEZOIDAL
GENERATORS.

THE BASIC OSCILLATOR

An oscillator can be thought of as an amplifier that provides itself (through feedback) with an input signal. By
definition, it is a nonrotating device for producing alternating current, the output frequency of which is
determined by the characteristics of the device. The primary purpose of an oscillator is to generate a given
waveform at a constant peak amplitude and specific frequency and to maintain this waveform within certain limits
of amplitude and frequency.

An oscillator must provide amplification. Amplification of signal power occurs
from input to output. In an oscillator, a portion of the output is fed back to sustain the input, as shown in
figure 2-1. Enough power must be fed back to the input circuit for the oscillator to drive itself as does a signal
generator. To cause the oscillator to be self-driven, the feedback signal must also be

Figure 2-1.—Basic oscillator block diagram.

2-2

REGENERATIVE (positive). Regenerative signals must have enough power to compensate for circuit losses
and to maintain oscillations.

Since a practical oscillator must oscillate at a predetermined frequency, a
FREQUENCY- DETERMINING DEVICE (FDD), sometimes referred to as a FREQUENCY-DETERMINING NETWORK (FDN), is needed.
This device acts as a filter, allowing only the desired frequency to pass. Without a frequency-determining device,
the stage will oscillate in a random manner, and a constant frequency will not be maintained.

Before
discussing oscillators further, let's review the requirements for an oscillator. First, amplification is required
to provide the necessary gain for the signal. Second, sufficient regenerative feedback is required to sustain
oscillations. Third, a frequency-determining device is needed to maintain the desired output frequency.

The basic oscillator requirements, in addition to the application, determine the type of oscillator to be used.
Let's consider some factors that account for the complexity and unique characteristics of oscillators.

Virtually every piece of equipment that uses an oscillator has two stability requirements, AMPLITUDE STABILITY and
FREQUENCY STABILITY. Amplitude stability refers to the ability of the oscillator to maintain a constant amplitude
in the output waveform. The more constant the amplitude of the output waveform, the better the amplitude
stability. Frequency stability refers to the ability of the oscillator to maintain its operating frequency. The
less the oscillator varies from its operating frequency, the better the frequency stability.

A constant
frequency and amplitude can be achieved by taking extreme care to prevent variations in LOAD, BIAS, and COMPONENT
CHARACTERISTICS. Load variations can greatly affect the amplitude and frequency stability of the output of an
oscillator. Therefore, maintaining the load as constant as possible is necessary to ensure a stable output.

As you should know from your study of transistor biasing, bias variations affect the operating point of the
transistor. These variations may alter the amplification capabilities of the oscillator circuits as well. A
well-regulated power supply and a bias-stabilizing circuit are required to ensure a constant, uniform signal
output.

As a result of changing temperature and humidity conditions, the value or characteristics of
components such as capacitors, resistors, and transistors can change. The changes in these components also cause
changes in amplitude and frequency.

Output power is another consideration in the use of oscillators.
Generally speaking, high power is obtained at some sacrifice to stability. When both requirements are to be met, a
low-power, stable oscillator can be followed by a higher-power BUFFER AMPLIFIER. The buffer provides isolation
between the oscillator and the load to prevent changes in the load from affecting the oscillator.

If the
oscillator stage must develop high power, efficiency becomes important. Many oscillators use class C bias to
increase efficiency. Other types of oscillators may use class A bias when a high efficiency is not required but
distortion must be kept at a minimum. Other classes of bias may also be used with certain oscillators.

SINE-WAVE OSCILLATOR

RC networks, LC tanks, and crystals may appear in sine-wave
oscillator circuits. An amplifier can be made into a sine-wave oscillator by providing regenerative feedback
through an RC network.

2-3

RC Network

Figure 2-2, view (A), shows the block diagram of an amplifier with an RC
network through which regenerative feedback is provided. The RC network also acts as the frequency-determining
device. View (B) shows a vector analysis of the signal E at various points in the circuit.

Figure 2-2A.—RC oscillator. AMPLIFIER WITH AND RC FEEDBACK NETWORK.

Figure 2-2B.—RC oscillator. VECTOR ANALYSIS

To analyze the operation of the circuit in view (A), assume that the amplifier is a common-emitter
configuration. The signal on the collector (M) is 180 degrees out of phase with the signal (input) on the base
(R). For the circuit to produce regenerative feedback, the RC network must provide a 180-degree phase shift of the
collector signal. When power is applied to the circuit, a noise voltage (noise contains many different
frequencies) will appear on the collector. This noise signal is represented by vector LM in view (B). As the
signal couples through C1 and across R1 (view (A)), a phase shift occurs. The voltage across R1 (ER1),
represented by vector LN, has been shifted in phase (about 60 degrees) and reduced in amplitude. The signal at
point N (view (A)) is then coupled to the next RC section (R2 and C2). Using the same size resistor and capacitor
as before will cause another 60-degree phase shift to take place. The signal at point P is the voltage across R2,
represented by vector LP. Now the signal at point P has been shifted about 120 degrees and its amplitude is
reduced still further. The same actions occur for the last section (R3 and C3). This signal experiences another
60-degree phase shift and has further amplitude reduction. The signal at point R (ER3) has been shifted 180
degrees and is represented by vector LR.

Notice that point R is the input to the base of the
common-emitter amplifier. Also, vector LR shows that the signal on the base is regenerative (aiding the circuit
operation). This meets the regenerative feedback requirement. An exact 60-degree phase shift per stage is not
required, but the sum of the three phase shifts must equal 180 degrees.

2-4

For a given RC network, only one frequency of the initial noise signal will be shifted exactly 180
degrees. In other words, the network is frequency selective. Therefore, the RC network is the frequency-
determining device since the lengths of the vectors and their phase relationships depend on frequency. The
frequency of oscillations is governed by the values of resistance and capacitance in these sections. Variable
resistors and capacitors may be used to provide tuning in the feedback network to allow for minor variations in
phase shift. For an RC phase-shift oscillator, the amplifier is biased for class A operation to minimize
distortion of the wave or signal.

LC Network

Some sine-wave oscillators use resonant circuits consisting of inductance and
capacitance. For example, recall the tank circuit in which a resonant circuit stores energy alternately in the
inductor and capacitor, producing a sine wave. You studied this action of the tank circuit in chapter 1.

If there were absolutely no internal resistances in a tank circuit, oscillations would continue indefinitely, as
shown in figure 2-3, view (A). Each resonant circuit does, however, contain some resistance which dissipates
power. This power loss causes the amplitude to decrease, as shown in views (B) and (C). The reduction of amplitude
in an oscillator circuit is referred to as DAMPING. Damping is caused by both tank and load resistances. The
larger the tank resistance, the greater the amount of damping. Loading the tank causes the same effect as
increasing the internal resistance of the tank. The effect of this damping can be overcome by applying
regenerative feedback.

Figure 2-3A.—Effects of damping.

Figure 2-3B.—Effects of damping.

Figure 2-3C.—Effects of damping.

2-5

Figure 2-4 shows a block diagram of a typical LC oscillator. Notice that the oscillator contains the
three basic requirements for sustained oscillations: amplification, a frequency-determining device, and
regenerative feedback.

Figure 2-4.—LC oscillator.

The amplifier supplies energy to begin what is known as the FLYWHEEL EFFECT. The flywheel effect is the
maintenance of oscillations in a circuit in the intervals between pulses of excitation energy. Recall that in
chapter 1 the tank circuit alternately stored energy in the inductor and capacitor. The LC network provides
initial oscillations. A portion of the output of the LC network is then returned to the input of the amplifier
through the regenerative-feedback network to sustain the oscillations.

When a tank circuit is used to
develop oscillations in an oscillator, the output frequency of the oscillator is primarily the resonant frequency
of the tank circuit and can be found by the formula:

Crystals

Another frequency-determining device is the CRYSTAL. The crystal may
be used with a tank circuit, or it may perform alone. Crystals exhibit a characteristic known as the PIEZOELECTRIC
EFFECT. The piezoelectric effect is the property of a crystal by which mechanical forces produce electrical
charges and, conversely, electrical charges produce mechanical forces. This effect is a form of oscillation
similar to the flywheel effect of a tank circuit.

The piezoelectric effect can be seen in a number of
crystal substances. The most important of these are the minerals quartz and Rochelle salt. Although quartz does
not exhibit the piezoelectric effect to the degree that Rochelle salt does, quartz is used for frequency control
in oscillators because of its greater mechanical strength. Another mineral, tourmaline, is physically strong like
quartz; but because it is more expensive, it is not used extensively as an FDD. This discussion will deal only
with the quartz crystal.

The crystals used in oscillator circuits are thin sheets, or wafers, cut from
natural or synthetic quartz and ground to a specific thickness to obtain the desired resonant frequency. The
crystals are mounted in holders, which support them physically and provide electrodes by which voltage is applied.
The holder must allow the crystals freedom for vibration. There are many different types of holders. One type is
shown in figure 2-5.

2-6

Figure 2-5.—Crystal holder.

The frequency for which a crystal is ground is referred to as the NATURAL RESONANT FREQUENCY of the
crystal. Voltage applied to the crystal produces mechanical vibrations which, in turn, produce an output voltage
at the natural resonant frequency of the crystal. A vibrating crystal can be represented by an equivalent
electrical circuit composed of capacitance, inductance, and resistance.

Figure 2-6, view (A), illustrates
the symbol of a crystal; view (B) shows an equivalent circuit for the crystal. View (C) shows an equivalent
circuit for the crystal and the holder; C1 represents the capacitance between the metal plates of the holder.

The Q (discussed in chapter 1) of a crystal is many times greater than that of an LC tank circuit. The
high Q is present because the resistance in the crystal is extremely small. Commercially produced crystals range
in Q from 5,000 to 30,000. The high Q causes the frequency stability to be much greater than thatof an
ordinary LC tank circuit. This is the reason a crystal is used in many sine-wave generator circuits.

Q-1. What are the two classifications of wave generators according to their output waveshapes?

Q-2.
What are the three networks used for frequency-determining devices?

Q-3. What is another name for
nonsinusoidal oscillators?

Q-4. What is a nonrotating device that produces alternating current?

Q-5. What are the three requirements necessary for oscillations to exist in a circuit?

SOLID-STATE LC OSCILLATORS

As you have just studied, a basic oscillator can be broken
down into three main sections: a frequency-determining device, an amplifier, and a feedback circuit. The
frequency-determining device in an LC oscillator is usually an LC tank circuit. Although the tank circuit is
normally found in the input
circuit of an oscillator (both electron tube and transistor), it sometimes appears in the output circuit. The
differences in magnitude of plate and collector currents and shunting impedances are considerations in the
designed locations of such tank circuits. In both solid-state and electron tube circuits, oscillations take
place in the tuned circuit. Both the electron tube and the transistor function primarily as electrical valves that
amplify and automatically deliver to the input circuit the proper amount of energy to sustain oscillations. In
both tube and transistor oscillators, the feedback circuit couples energy of the proper amount and of the correct
phase from the output to the input circuit to sustain oscillations.

FEEDBACK

Let's review what you have studied up to this point concerning feedback. Feedback is the process of
transferring energy from a high-level point in a system to a low-level point in a system. This means transferring
energy from the output of an amplifier back to its input. If the output feedback signal opposes the input signal,
the signal is DEGENERATIVE or NEGATIVE FEEDBACK. However, if the feedback aids the input signal, the feedback is
REGENERATIVE or POSITIVE FEEDBACK. Regenerative or

2-8

positive feedback is one of the requirements to sustain oscillations in an oscillator. This feedback
can be applied in any of several ways to produce a practical oscillator circuit.

TYPES OF FEEDBACK

Chapter 1 described the resonant or tank circuit and how a sinusoidal signal is generated by the action of an
inductor and a capacitor. The feedback signal is coupled from this circuit by either of two means. The first
method is to take some of the energy from the inductor. This can be done by any one of the three ways shown in
figure 2-7, views (A), (B), and (C). When an oscillator uses a TICKLER COIL, as shown in view (A), it is referred
to as an ARMSTRONG OSCILLATOR. When an oscillator uses a tapped coil (view (B)) or a split coil (view (C)), it is
referred to as a HARTLEY OSCILLATOR. The second method of coupling the feedback signal is to use two capacitors in
the tank circuit and tap the feedback signal between them. This is shown in view (D). An oscillator using this
method is referred to as a COLPITTS OSCILLATOR. Each of these particular oscillators is named after the person who
originally designed them.

Figure 2-7.—Feedback signals.

CONFIGURATION OF OSCILLATORS

Any of the three basic amplifier configurations (common collector, common base, or common emitter)
described in NEETS, Module 7, Introduction to Solid-State Devices and Power Supplies, Chapter2, may be used
for the oscillator circuit. However, certain considerations in the application of the circuit, such as the
operating frequency and output power required, usually determine which of the three configurations is to be used.
The three basic configurations are shown in figure 2-8, views (A), (B), and(C).

Figure 2-8A.—Basic configurations. COMMON-COLLECTOR CONFIGURATION

2-9

Figure 2-8B.—Basic configurations. COMMON-BASE CONFIGURATION.

Figure 2-8C.—Basic configurations. COMMON-EMITTER CONFIGURATION.

COMMON-COLLECTOR CONFIGURATION

Since there is no phase reversal between the
input and output circuits of a common-collector configuration, the feedback network does not need to provide a
phase shift. However, since the voltage gain is less than unity and the power gain is low, the common-collector
configuration is very seldom used in oscillator circuits.

COMMON-BASE CONFIGURATION

The power gain and voltage gain of the common-base configuration are high enough to give satisfactory operation in
an oscillator circuit. The wide range between the input resistance and the output resistance make impedance
matching slightly harder to achieve in the common-base circuit than in the common-emitter circuit. An advantage of
the common-base configuration is that it exhibits better high- frequency response than does the common-emitter
configuration.

COMMON-EMITTER CONFIGURATION

The common-emitter configuration has high power gain and is
used in low-frequency applications. For the energy which is fed back from the output to be in phase with the
energy at the input, the feedback network of a common-emitter oscillator must provide a phase shift of
approximately 180 degrees. An